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+This script can be used to implement persistent leases on openWRT, DD-WRT
+etc. Persistent leases are good: if the lease database is lost on a
+reboot, then it will eventually be restored as hosts renew their
+leases. Until a host renews (which may take hours/days) it will
+not exist in the DNS if dnsmasq's DDNS function is in use.
+
+*WRT systems remount all non-volatile fileystems read-only after boot,
+so the normal leasefile will not work. They do, however have NV
+storage, accessed with the nvram command:
+
+/usr/lib #  nvram
+usage: nvram [get name] [set name=value] [unset name] [show]
+
+The principle is that leases are kept in NV variable with data
+corresponding to the line in a leasefile:
+
+dnsmasq_lease_192.168.1.56=3600 00:41:4a:05:80:74 192.168.1.56 * *
+
+By giving dnsmasq the leasefile-ro command, it no longer creates or writes a
+leasefile; responsibility for maintaining the lease database transfers
+to the lease change script. At startup, in leasefile-ro mode,
+dnsmasq will run
+
+"<lease_change_script> init" 
+
+and read whatever that command spits out, expecting it to
+be in dnsmasq leasefile format.
+
+So the lease change script, given "init" as argv[1] will 
+suck existing leases out of the NVRAM and emit them from
+stdout in the correct format.
+
+The second part of the problem is keeping the NVRAM up-to-date: this
+is done by the lease-change script which dnsmasq runs when a lease is
+updated. When it is called with argv[1] as "old", "add", or "del"
+it updates the relevant nvram entry.
+
+So, dnsmasq should be run as :
+
+dnsmasq --leasefile-ro --dhcp-script=/path/to/lease_update.sh
+
+or the same flags added to /etc/dnsmasq.conf
+
+
+
+Notes: 
+
+This needs dnsmasq-2.33 or later to work.
+
+This technique will work with, or without, compilation with
+HAVE_BROKEN_RTC. Compiling with HAVE_BROKEN_RTC is
+_highly_recommended_ for this application since is avoids problems
+with the system clock being warped by NTP, and it vastly reduces the
+number of writes to the NVRAM. With HAVE_BROKEN_RTC, NVRAM is updated
+only when a lease is created or destroyed; without it, a write occurs
+every time a lease is renewed.
+
+It probably makes sense to restrict the number of active DHCP leases
+to an appropriate number using dhcp-lease-max. On a new DD_WRT system,
+there are about 10K bytes free in the NVRAM. Each lease record is
+about 100 bytes, so restricting the number of leases to 50 will limit
+use to half that. (The default limit in the distributed source is 150)
+
+Any UI script which reads the dnsmasq leasefile will have to be
+ammended, probably by changing it to read the output of 
+`lease_update init` instead.
+ 
+
+Thanks:
+
+To Steve Horbachuk for checks on the script and debugging beyond the 
+call of duty.
+
+
+Simon Kelley
+Fri Jul 28 11:51:13 BST 2006
+
+
+
+
+